1. Field of the Invention
The present invention relates to pollution control methods and apparatus, and in particular to methods and apparatus for reducing pollutant emissions from spreader-stoker-fired furnaces and fluidized bed combustors by removing fines from the material to be combusted.
2. The Prior Art
For centuries, man has relied upon the combustion of combustible materials, such as coal and wood, to provide heat energy. One of the most common methods for harnessing this heat energy is to use the heat energy to generate steam. Over the years, different types of furnaces or boilers have been developed for the combustion of coal, wood, and other combustible materials.
One type of furnace, the stoker-fired furnace, was developed to burn relatively large particles of coal, up to about 1.5 inches in diameter. Later, another type of furnace, the pulverized coal-fired furnace, was developed for burning much smaller coal particles, e.g., where about 70% of the coal particles pass through a 200 mesh screen. Pulverized coal-fired furnaces have large steam generating capacities and are thus typically used in steam generating installations where at least 500,000 pounds of steam per hour are required. The electric power generating industry has been one of the largest users of pulverized coal-fired furnaces, since large amounts of steam are required for the production of electric energy.
Because of the small particle sizes of coal which are used in the pulverized coal-fired furnaces, expensive pulverizing steps are necessarily employed to reduce the particle size of the coal. Moreover, pulverized coal-fired furnaces involve extensive capital outlays. As a result, whenever practical, those skilled in the art prefer to use stoker-fired furnaces. Stoker-fired furnaces have especially found utility in smaller operations where the steam generating capacity of the stoker-fired furnace is sufficient to meet the needs of the operation.
In the late 1940's and early 1950's, there was a large decline in the demand for commercial and industrial solid fuel-fired systems (such as the stoker-fired and pulverized coal-fired systems) due to the wide-spread availability of relatively cheap oil and natural gas sources. In the 1960's, the stoker-fired and pulverized coal-fired systems became even less attractive because of their relatively high pollutant emissions when compared with the oil and gas-fired systems. Thus, the oil and gas-fired systems substantially replaced the coal-fired systems until the gas and oil petroleum-based fuels became less plentiful during the 1970's. The petroleum shortage experienced during the 1970's has caused industry to begin to look once again to the coal-fired and other solid fuel-fired systems.
In recent years, considerable emphasis has been given to solid fuel research, particularly in the area of burning solid fuels such as coal and wood without excessive pollutant emissions. As the costs of oil and gas continue to escalate, the utilization of solid fuel systems (such as coal-fired systems) will continue to increase. In particular, the use of stoker-fired systems is increasing due to the substantial savings involved when the larger coal particles are introduced into the furnace without expensive pulverizing steps as are necessary for the pulverized coal-fired processes.
One type of stoker-fired furnace, and undoubtedly the most popular type, is the spreader-stoker-fired furnace. The spreader-stoker-fired furnace is characterized in that it has a paddle wheel-type mechanism or air jet for flinging the coal particles into the furnace such that the coal particles are suspended in and travel through a suspension or overthrow region within the furnace for an appreciable period of time before falling onto a grate located at the bottom of the furnace. This suspension of the coal particles within the suspension region of the spreader-stoker-fired furnace is commonly referred to as the "suspension phase." In typical spreader-stoker-fired furnace systems, a portion of the coal is combusted in the suspension phase, before reaching the grate. Coal particles which are not burnt during their descent in the suspension phase, come to rest against the grate and form a burning fuel bed in a bed region of the furnace. Other coal particles are entrained by the flow of gases within the furnace and are not combusted in either the suspension or bed regions, but rather escape uncombusted in the furnace effluent. The grate on which the burning fuel bed resides moves at a very slow rate, e.g., from about 5 to 40 feet per hour, and eventually dumps the combustion by-products (namely, residual ash) into an ash pit. Alternatively, the grate may be stationary but have the capability of being dumped at periodic intervals to remove the bed of accumulated ash.
One reason for the popularity of the spreader-stoker-fired furnace is its high superficial grate heat release rates of up to 750,000 BTU/hr-ft.sup.2 and its low inertia due to nearly instantaneous fuel ignition upon increased firing rate. This high superficial grate heat release is obtained because of the relatively uniform distribution of the coal particles in the burning fuel bed on the grate, the relatively small depth of the layer of coal particles on the grate, and the intense combustion during the suspension phase above the burning fuel bed. The low inertia allows the spreader-stoker-fired furnace to respond rapidly to load fluctuations in steam demand, and hence in boiler load, which are common in industrial applications.
In addition, spreader-stoker-fired furnaces are capable of firing fuels with a wide range of burning characteristics, including coals with caking tendencies, since rapid surface heating of the coal in the suspension phase destroys the caking propensity. Additionally, little or no fuel preparation is required for spreader-stoker firing of coal; if needed, the coal can be crushed to particle sizes of about 1.5 inches or less in diameter and directly fired. In other types of stoker-fired furnaces, the coal particles are typically introduced directly onto the burning fuel bed at the bottom of the furnace without experiencing a suspension phase.
During the combustion of solid fuels (such as coal), nitrogen which is bound primarily in heterocyclic ring structures is liberated as CN fragments which subsequently react to form nitrogen gas (N.sub.2) or nitrogen oxide pollutants. The nitrogen oxide pollutants, generally designated NO.sub.x, are primarily in the form of nitric oxide (NO) and nitrogen dioxide (NO.sub.2). While the nitrogen gas emissions are relatively harmless, the NO.sub.x emissions are highly toxic. Nitrogen dioxide is an especially dangerous pollutant since NO.sub.2 as well as other pollutants such as SO.sub.2 and SO.sub.3, are often responsible for what is known as acid rain. Even if the NO.sub.x emissions are in the form of NO, which is the favored nitrogen oxide formed in most combustion processes, NO is readily oxidized in the atmosphere to NO.sub.2.
Although spreader-stoker-fired furnaces have been thought to be more efficient than other stoker-fired furnaces due to improved exposure of coal particles to oxygen during the suspension phase, excessive NO.sub.x emissions from spreader-stoker-fired furnaces have been experienced. These undesirable NO.sub.x emissions may exceed currently proposed governmental standards, and therefore may tend to discourage the use of spreader-stoker-fired furnaces.
Other pollutant emissions characteristic of spreader-stoker-fired furnaces include particulate emissions. Particulate emissions become a particular problem in spreader-stoker-fired furnaces since the solid fuel or coal particles are suspended for an appreciable period of time during the suspension phase where they are contacted by the rising flow of combustion gases and a relatively forceful stream of air. Such contact between the particles and the flow of gases during the suspension phase increases the amount of coal, ash, and other particulates which are entrained in the furnace effluent.
In view of the wide-spread popularity of the spreader-stoker-fired furnace for the combustion of coal, wood, and other combustible materials, it would be a significant advancement in the art to provide a method and apparatus for reducing pollutant emissions, and in particular for reducing NO.sub.x and particulate emissions, from such spreader-stoker-fired systems. Such a method and apparatus are disclosed and claimed herein.